Method of producing wet-strength papers



United States Patent Thomas J. Brennan, Philadelphia, and Louis E. Kelley,

y s e. at, ass u o Rchm & aa C p y,

Philadelphia, Pa., a corporation of Delaware N'o Drawing Filed. Jan- 24, 1958. n N 710,877 7 C aimsrill-+1.

The present invention is concerned with a new and improved process for the production of papers having improved t-streng hs- It has heretotfore been proposed inv McLaughlin United t s Patent .7 5.229 ancho linear p lymers c nt ug c y te units to the. fibers in p p rs by ca nic agents to improve. various dry-strengths such as the burst, ear. and t n i e-s reuat -v O of. he adv n ge f ha pr ced r as ha the wet-s ngth. of the Pap s rea ed a no ppre iably inc ase so h o e r c e y was p i e- It has now e n ou d that. the direc imp egna ion of certainv pape n a u a tia ly ry stat and und r. c rtain conditions with aqueous solutions of certain polymers containing ammonium carboxylate. units serves, to produce an unexpected and surprising increase in wetstrength when the amount of polymer deposited is in the range of 2% to 8% based on the weight of the fiber.

The types of papers with which the present process is concerned are those-having a basis weight between 10 and 60 lbs. for a 500,sheet ream in which the sheets are 24" x 36". that they have Canadian F-reeness values in the range of 300 to 400. They may be sized papers and particularly those sized with rosin in conventional manner using alums such as the; conventional papermakers alum. The amount of rosin sizing may be that which is customary, namely from 0.2% to 2% by weight of the rosin based on the weight; of fibers in the paper. These. Pap rsare adapted when finished to be employed as wrapping papers (when prepared from sized stock.) or as. tissue papersiueluding toilet tissue, toweling, creped or not, diapers, and for other moisture-absorbent uses (when prepared from unsized stock).

The linear polymers the ammonium salts of which have been found to impart the wet-strength to such papers when the latter are impregnated in aqueous solutions of the polymer salts are those which contain 25% to 6.0% by weight of units derived from an acid selected from the group'consisting of methacrylic acid, acrylic acid, and itaconic acid. The balance of the polymer is formed of polymerized units of acrylonitrile or esters of acrylic or methacrylic acid. For example, the polymer may contain up to 75% by weight of an ester of acrylic acid with a lower alcohol ha i g f om t 4. c on atoms, s ch as.- methyl acrylate, t yl y at or buty acrylate. Besides the lower alkyl esters of acrylic acid (that is, instead, thercot' or in addition thereto}, the cm polymer may contain units. of higher alcohol esters of acrylic acid such as cyclohexyl acrylate, n-hexyl acrylate, amyl acrylate, Zethyl-hexyl acrylate, t-octyl. acrylate, decyl acrylate, dod'ecyl acrylate, hexadecyl acrylate, or octadecyl acrylate. Besides the ester of acrylic acid, the copolymer may contain units. of acrylonitrile, or. of lower alkyl esters of methacrylic acid, such as methyl methacrylate, ethyl methacrylate, or butyl methacrylate, or even higher alkyl esters; of acrylic acid such, as; the cyclohexyl and other alkyl methacrylates corresponding to the higher alkyl esters of acrylic acid mentioned herein,- above.

It i al o. ess ntial that he c p ymer have. a high molecular weight. Since the molecular weights of co,- polymers containing a large proportion of acid units are The papers must also be highly beaten so I Patented Sept. 5, 1961 generally diflicult to measure because of the viscosity efiects produced by the carboxylate units, the most satisfactory way of expressing the range of molecular weights is by reference to a viscosity of the ammonium salt of the copolymer in solutions thereof in water at a concentration of 10% and. at 25 C. Such viscositieswhen measured with a Brookfield viscometer at 60 rpm. are in the range of 12 to 40 poises. It is. generally believed that the molecular weights of these polymers are at least 500,000 (viscosity average) and may even. have a minimum viscosity average molecular weight of at least one million. Many of, the polymers. with which the present invention is concerned have molecular weights as. high as several million. In order to produce such high molecular weight polymers, the process resorted. to for their preparation may either be an emulsion or a suspension polymerization procedure. When the emulsion procedure is employed, the'emulsifier may be used in. a proportion of about /2%.to 6% based on the weight-of monomers (total). For suspension polymerization, a lesser amount of emulsifier and frequently aless effective emulsifier is generally employed. In either case conventional aqueous emulsion or suspension polymerization techniques are employed. Preferably, water-soluble initiators are used such as the alkali metal or ammonium persulfates in amounts from 0.10% to 3% on the weight of the monomers. Preferably, a redox system is used in. which the persulfate or like. initiator is used in conjunction with a reducing agent such as sodium metabisulfite in about the same proportion as the initiator. A promoter, such as small proportions of metalv ions such as. copper, iron, or the like, may be used. The polymeriz-ation may be. effected as. a batch system or by continuous addition of the monomers. to the aqueous solution containing initiator, reducing agent and promoter. Generally, it is unnecessary to heat the polymerization system, and in many cases it may be desirable to cool the. system to control the temperature and maintain it between room temperature and 85." C. Agitation during the polymerization is generally desirable but may be omitted. The amount of water may be. selected. at. the beginning of the polymerization procedure with reference to. the amount 0t comonomers to be copolyrnerized so that any desired concentration on the order of. 25% to. 50% or more of copolymeris obtained in dispersed condition in the reaction vessel when, an emulsion, procedure is. employed. If the suspension procedure is em.- ployed, small granules or beads of the polymer form in thesystem.

When the emulsion technique is employed, it ismerel-y necessary to add ammonia to the aqueous dispersion of the emulsion copolymer until a pH of 8 to 10 or more is reached. Such neutralization forms the salt of the oopolymer and results in a colloidal solution of the ammonium salt thereby obtained within the water of the system. In the case of suspension polymerization, the granules, may be separated from the polymerization medium and then neutralized with ammonia in a suitable. solution or the ammonia may be added to the polymerization system. containing the granules without the'separation thereof. This results in neutralization to form the ammonium salt. which dissolves in the water.

The solution of the. ammonium salt of the polymer is originally prepared or diluted to a concentration of 2% to. 10% by weight of the ammonium salt. in the solution for; application. The aqueous solution of the. ammonium salt of the. copolymer is applied to the paper by simply dipping or immersing the paper therein while the solution is at a temperature of 70 to F. Advantageously, the paper may proceed continuously through such aqueous polymer salt solution. through such a solution in a size press immediately follow- For example, it may be. passed ing the normal paper drying drums. Of course, the paper proceeds from the size box into a final drier. When unsized papers pass through the polymer salt solution, there is approximately 125% wet pickup; whereas when a sized paper is employed, the wet pickup is generally about 20% to 50% cold or up to 80% if heated so that the concentration of the polymer salt solution employed with sized papers should be at least about 2% to 4%.

The paper may be formed from a wide variety of materials. Thus, it may be a kraft pulp, rag pulp, soda, sulfate, groundwood, sulfite pulp, or alpha pulp. Besides employing the normal wood pulp fibers, the paper may be formed of cotton-linters and, if desired, the wood or cotton fibers may be used in admixture with fibers from other sources such as jute, hemp, sisal, rags, strings, chopped canvas, and so on.

The finished paper products may be employed as filter papers, disposable diaper fillers, toilet tissue, paper toweling, bibs, and various wrapping materials.

One of the advantages of the procedure of the present invention is the fact that the normal drying equipment employed in conventional papermarking machinery is adequate for drying the paper after impregnation with the water-soluble copolymer salt. Hence, the process can be employed as an on-machine size press coating operation. However, if desired, the paper may be treated in an ofi-machine converting process by passing the paper through a saturating bath (size tub) and then dried and/or creped on a drum drier or other drying equipment. When the polymers are prepared by the suspension technique which is the preferred technique, relatively little emulsifier is present and the application of the copolymer salt actually serves not only to improve wet-strength but also sizes the paper when unsized paper is the initial starting material. In other words, the ammonium copolymer salt after drying down to the insoluble form on the impregnated sheet reduces the penetration of water and/ or ink; whereas the presence of a large proportion of emulsifier as would be the case when an emulsion polymerization technique is employed would ordinarily not be capable of reducing the penetration by water. Most of the wet-strength resins conventionally employed require either a high temperature cure (in excess of the temperature available in the conventional paper-drying machinery) or require ageing for as much as one month in order to develop a full wetstrength obtainable therefrom. Such wet-strength resins of thermosetting aminoplast type also need a low pH on the acid side to facilitate the cure or development of the wet-strength on ageing. In contrast with this, the polymer salt solutions of the present invention are applied at a pH of about 8 to 10 where less likelihood of corrosion on the machinery is encountered.

Another advantage of the alkaline polymer salt solutions of the present invention is the fact that they need not be at higher temperatures than 140 F. or even above 70 F. to provide good impregnation and pickup, whereas the customary thermosetting aminoplast resins generally have to be at a temperature of 180 F. at the point of impregnation in order to provide efficient pickup by the paper.

In general, the copolymer salts of the present invention provide an improvement in wet-strength immediately on drying which is from 50% to 100% greater than the wetstrength obtained with conventionally employed thermosetting aminoplast resins even after 28 days of ageing required for the substantially full development of wetstrength with such aminoplast resins.

The conventional thermosetting aminoplast resins are applied at about 14% solids at about 180 F. at an acid pH of about 4.5 with alum or hydrochloric acid. They have a tendency to gel at this temperature and must be promptly used. Batches of as much as 200 gallons are made up at a time and any left over after a batch wetstrength paper run must be thrown out. In contrast with this, the aqueous polymer salt solutions of the present invention are stable and can be stored indefinitely so that they can be saved for the next wet-strength batch conversion operation.

Typical thermosetting aminoplast types of wet-strength resins when applied to sized papers either reduce or destroy the sizing effects. The compositions of the present invention either have no effect on the size of the sized paper'in the case of an emulsion copolymer or, when a suspension type of polymer is employed for producing the copolymer salt solution, the sizing is increased.

In the following examples which are illustrative of the present invention, the parts and percentages are by weight unless otherwise noted.

EXAMPLE 1 An aqueous solution containing 5% of an ammonium salt of a copolymer of 50% by weight of ethyl acrylate, 17% of methyl methacrylate, and 33% of methacrylic acid (having a viscosity of 37 poises at 25 C. and 10% concentration) was applied at 135 F. to a rosin-sized bag paper obtained trom a pulp having a Canadian freeness of 300 ml. by passing the paper through the solution and then through rubber squeeze rolls. The impregnated paper picked up 4% of copolymer (on fiber) and was dried at room temperature. Similarly a solution of the same polymer salt was applied at 2.5% concentratlon providing 1.9% copolymer on the fiber weight of the dried sheet. The wet strengths (machine direction) were measured after a 24-hour soak in water at room temperature. The results are given in the following Table I. A commercial cationic urea-formaldehyde condensate having a pH of 4.5 was also applied at 5% and 2.5% concentrations (giving 2.5% and 1.5% pickups) and tested after a 28-day ageing for comparisons.

Table I Ooncen- Wet Tensile Saturant; tration, pH (MD), lbs./

percent in. width EXAMPLE 2 An aqueous 5% solution of an ammonium salt of 75% methyl methacrylate and 25% of methacrylic acid (having a viscosity of 20 poises at 10% concentration and 25 C.) was applied to a rosin-sized bag paper as m Example 1. The polymer pickup was 3.95% on fiber. The wet strength after a 24-hour water-soak was 8.0 lbs. per inch width. EXAMPLE 3 A saturating paper (unsized) having a basis weight of 32 pounds and obtained from a pulp having a Canadian freeness of 400 ml. was impregnated as in Example 1 with a 5% aqueous solution of the copolymer salt of Example 2. The wet strength was 4.9 lbs. per inch width. The untreated sheet disintegrates completely in the soak.

EXAMPLE 4 The procedure of Example 3 was repeated substituting for the copolymer salt the ammonium copolymer salt of Example 1. The wet-strength was 9.4 lbs. per inch width (measured after a 2Ahour soak).

EXAMPLE 5 acrylate and 40% of methacrylic acid (having a viscosity at concentration and 25 C. of 36 poises). The paper after drying contained 3.65% of copolymer salt. The wet strength was raised to 7.2 lbs. per inch width.

EXAMPLE 6 The procedure of Example 5 was repeated substituting for the copolymer salt an ammonium salt of a copolymer of 40% methyl methacrylate and 60% of methacrylic acid at 5% concentration. The copolymer salt at 10% concentration and 25 C. had a viscosity of 20 poises. The pickup was 7.5% and the wet strength was 9.9 lbs. per inch width.

It is to be understood that changes and variations may be made without departing from the spirit and scope of the invention as defined by the appended claims.

We claim:

1. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24 x 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a water-insoluble copolymer of a mixture of 25% to 60% by weight of an acid selected from the group consisting of methacrylic acid, acrylic acid, and itaconic acid, and at least one other comonomer selected from the group consisting of acrylonitrile, esters of acrylic acid with an alcohol selected from the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, and esters of methacrylic acid with an alcohol selected from the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being free of any cationic agent, and subsequently drying the impregnated paper.

2. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry paper prepared from fibers beaten to a Canadian reeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" X 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a water-insoluble copolymer of a mixture of methyl methacrylate, ethyl acrylate, and 25% to 60% by weight of met-hacrylic acid, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being iree of any cationic agent, and subsequently drying the impregnated paper.

3. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a water-insoluble copolymer of a mixture of ethyl acrylate and 25 to 60% by weight of methacrylic acid, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being free of any cationic agent, and subsequently drying the impregnated paper.

4. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" X 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a water-insoluble copolymer of a mixture of methyl methacrylate and 25% to 60% by weight of methacrylic acid, the ammonium salt having a viscosity from 12 to 40 poises at 10% con- 6 centration in water and at 25 C., the paper and aqueous solution being iree of any cationic agent, and subsequently drying the impregnated paper.

5. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry, unsized paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a water-insoluble copolymer of a mixture of 25 to 60% by weight of an acid selected from the group consisting of methacrylic acid, acrylic acid, and itaconic acid, and at least one other comonomer selected from the group consisting of acrylonitrile, esters of acrylic acid with an alcohol selected from the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, and esters of methacrylic acid with an alcohol selected from the group consisting oi cyclohexanol and alkanols having from 1 to 18 carbon atoms, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being free of any cationic agent, and subsequently drying the impregnated paper. 7

6. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry, rosin-sized paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous solution containing 2% to 10% by weight of an ammonium salt of a waterinsoluble copolymer of a mixture of 25 to 60% by weight of an acid selected from the group consisting of methacrylic acid, acrylic acid, and itaconic acid, and at least one other comonomer selected from the group consisting of acrylonitrile, esters of acrylic acid with an alcohol selected from the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, and esters of methacrylic acid with an alcohol selected from .the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being free of any cationic agent, and subsequently drying the impregnated paper.

7. A method for producing a wet-strength paper which consists of the steps of impregnating a substantially dry paper prepared from fibers beaten to a Canadian freeness between 300 and 400 and having a basis weight of 10 to 60 lbs. per ream of 500 sheets having a size of 24" x 36" with an aqueous solution containing 5 to 10% by weight of an ammonium salt of a water-insoluble copolymer of 25 to 60% by weight of an acid selected from the group consisting of methacrylic acid, acrylic acid, and itaconic acid, and at least one other comonomer selected from the group consisting of acrylonitrile, esters of acrylic acid with an alcohol selected from the group consisting of cyolohexanol and alkanols having from 1 to 18 carbon atoms, and esters of methacrylic acid with an alcohol selected from the group consisting of cyclohexanol and alkanols having from 1 to 18 carbon atoms, the ammonium salt having a viscosity from 12 to 40 poises at 10% concentration in water and at 25 C., the paper and aqueous solution being 'free of any cationic agent, and subsequently drying the impregnated paper.

References Cited in the file of this patent UNITED STATES PATENTS 2,661,309 Azorlosa Dec. 1, 1953 2,754,280 Brown et al July 10, 1956 2,765,228 Jordan Oct. 2, 1956 2,765,229 McLaughlin Oct. 2, 1956 2,790,735 McLaughlin Apr. 30, 1957 2,790,736 McLaughlin Apr. 30, 1957 

1. A METHOD FOR PRODUCING A WET-STRENGTH PAPER WHICH CONSISTS OF THE STEPS OF IMPREGNATING A SUBSTANTIALLY DRY PAPER PREPARED FROM FIBERS BEATEN TO A CANADIAN FREENESS BETWEEN 300 AND 400 AND HAVING A BASIS WEIGHT OF 10 TO 60 LBS. PER REAM OF 500 SHEETS HAVING A SIZE OF 24"X36" WITH AN AQUEOUS SOLUTION CONTAINING 2% TO 10% BY WEIGHT OF AN AMMONIUM SALT OF A WATER-INSOLUBLE COPOLYMER OF A MIXTURE OF 25% TO 60% BY WEIGHT OF AN ACID SELECTED FROM THE GROUP CONSISTING OF METHACRYLIC ACID, ACRYLIC ACID, AND ITACONIC ACID, AND AT LEAST ONE OTHER COMONOMER SELECTED FROM THE GROUP CONSISTING OF ACRYLONITRILE, ESTERS OF ACRYLIC ACID WITH AN ALCOHOL SELECTED FROM THE GROUP CONSISTING OF CYCLOHEXANOL AND ALKANOLS HAVING FROM 1 TO 18 CARBON ATOMS, AND ESTERS OF METHACRYLIC ACID WITH AN ALCOHOL SELECTED FROM THE GROUP CONSISTING OF CYCLOHEXANOL AND ALKANOLS HAVING FROM 1 TO 18 CARBON ATOMS, THE AMMONIUM SALT HAVING A VISCOSITY FROM 12 TO 40 POISES AT 10% CONCENTRATION IN WATER AND AT 25*C., THE PAPER AND AQUEOUS SOLUTION BEING FREE OF ANY CATIONIC AGENT, AND SUBSEQUENTLY DRYING THE IMPREGNATED PAPER. 